Coding Challenge Practice - Question 88

Source: Dev.to

Overview

Implement an EventEmitter that allows subscribing to events, emitting events with arguments, and releasing specific subscriptions.

Boilerplate

class EventEmitter {
  subscribe(eventName, callback) {}

  emit(eventName, ...args) {}
}

Each event name maps to an array of listener functions:

this.events = {};

Subscribe Method

• If the event does not exist, create an empty array for it.
• Append the callback to the array (duplicates are allowed).
• Return a subscription object with a release method that removes only this callback.

subscribe(eventName, callback) {
  if (!this.events[eventName]) {
    this.events[eventName] = [];
  }
  this.events[eventName].push(callback);

  let released = false;

  return {
    release: () => {
      if (released) return;
      released = true;

      const listeners = this.events[eventName];
      if (!listeners) return;

      const index = listeners.indexOf(callback);
      if (index !== -1) {
        listeners.splice(index, 1);
      }
    }
  };
}

Emit Method

• Retrieve the listeners for the given event.
• If none exist, exit.
• Call each listener with the provided arguments.

emit(eventName, ...args) {
  const listeners = this.events[eventName];
  if (!listeners) return;

  listeners.forEach(callback => {
    callback(...args);
  });
}

Complete Implementation

class EventEmitter {
  constructor() {
    this.events = {};
  }

  subscribe(eventName, callback) {
    if (!this.events[eventName]) {
      this.events[eventName] = [];
    }
    this.events[eventName].push(callback);

    let released = false;

    return {
      release: () => {
        if (released) return;
        released = true;

        const listeners = this.events[eventName];
        if (!listeners) return;

        const index = listeners.indexOf(callback);
        if (index !== -1) {
          listeners.splice(index, 1);
        }
      }
    };
  }

  emit(eventName, ...args) {
    const listeners = this.events[eventName];
    if (!listeners) return;

    listeners.forEach(callback => {
      callback(...args);
    });
  }
}